This Recommended Practice had defined a test procedure and process which gave a reasonably complete description of the efficiency and performance of a 20th century automatic transmission. With the introduction of electronic controls, the number of parameters which control the transmissionâs operation have expanded extensively and these need to be defined for each test. The interaction between the transmission, the environment, the engine, the cooling and other systems have also made test definition very difficult and complex. Finally, the diversity, the rapid changes, and the component complexity introduced by electric and hybrid drive system combine with the above difficulties to make it impractical to define a Recommended Practice which could provide a meaningful description of performance and efficiency of 21st century automotive transmissions. The existing Recommended Practice J651 is being stabilized to preserve the test procedures for future reference.